The present invention relates generally to a method and apparatus for cooling extrusion articles, and more specifically to substantially vaporizing a liquid cryogen and then circulating the vaporized cryogen through a cooling chamber, through a cooling chamber including sizing and/or calibration tools, through a hollow in the article itself or a combination of the aforementioned to cool an extrudate. The invention is particularly useful as an extrusion chiller, and may also be utilized for chilling foods. Additionally, many other applications of the invention will become apparent to those skilled in the art upon a review of the following specification and drawings.
Certain continuously extruded materials, e.g., rubber products, plastic products, metal products, wood composites, must be cooled after passing through the extrusion operation in order to prevent deformation. In conventional extrusion operations, the extruded materials, be it hose, pipe, rod, bar or any other shape may deform from its own weight if the temperature was not decreased rapidly after leaving the extruder. Cooling the product rapidly creates at least a minimum amount of rigidity in the extrudate such that the manufacturer can cut, stack or otherwise handle the extrudate without unwanted deformation. If the product is not cooled effectively and quickly, the resultant deformation can lead to excessive rates of rejection of the manufactured or extruded product. Further, the rate at which the extrudate is cooled directly affects the rate at which product may be produced. In other words, the faster an extrudate is cooled, the faster the end product can be produced.
Historically, cooling water systems have been utilized as the primary medium for cooling articles, including extrusions. For example, conventional extrusion chilling systems employ a xe2x80x9ccoolingxe2x80x9d chamber downstream from the extruder. The extrusion is fed through the cooling chamber, wherein the extrusion can be sprayed with water, or partially/fully submerged in water in order to chill the extrusion. Various other components may also be included in such systems, such as a vacuum sizing chamber intermediate the extruder and the cooling chamber. The vacuum sizing chamber can be used for both solid and hollow extrusions and employs an external vacuum pump to create a vacuum to assist the extrusion in maintaining its shape while it cools. Water can also be used in the vacuum chamber to cool the extrusion while the vacuum supports the shape. However, cooling water systems have several drawbacks. Many products are adversely affected if contacted with water. Thus, extra care must be taken to avoid such occurrences. Extrusion speeds are limited because the cooling water generally has a well defined heat transfer capability and thus can only cool the fresh extrudate in accordance therewith. In practice, an optimum cooling temperature of approximately 50xc2x0 F. is achievable from a cost-effective standpoint, which limits the manufacturer""s ability to cool extrusions quickly. Additionally, cooling water systems require excessive floor space and also require treatments or special additive packages to prepare and maintain proper water chemistry, as well as to prevent scaling and bacterial growth, which add significantly to the cost thereof.
Coolant mediums other than water which have been used in cooling processes can be referred to collectively as refrigerants, including cryogens. Cryogens include liquid nitrogen, liquid carbon dioxide, liquid air and other refrigerants having normal boiling points substantially below minus 50xc2x0 F. (xe2x88x9246xc2x0 C.). Prior art methods of cooling articles using cryogens disclose the benefits of fully vaporizing a cryogen into a gaseous refrigerant prior to contact with the articles to be cooled. Cryogens due to their extremely low boiling point, naturally and virtually instantaneously expand into gaseous form when dispersed into the air. This results in a radical consumption of heat. The ambient temperature can be reduced to hundreds of degrees below zero (Fahrenheit) in a relatively short time, and much quicker than may be realized with a conventional cooling water system. The extreme difference in vaporized cryogen and the extruded product allows the manufacturer to quickly cool an extrudate.
However, prior methods of cryogenic cooling fail to realize the advantages, both in increased efficiency and in improved system control, that can be achieved by utilizing forced gas convection in combination with nitrogen or any other refrigerant. Some disadvantages of prior art cryogenic cooling systems include lower efficiency and limited options for controlling the cooling process. Such systems generally rely exclusively on the cooling effect of the refrigerant, to lower the ambient temperature and chill the article. Although prior art methods utilize forced convection to ensure complete vaporization of the cryogen, no methods use forced gas convection to control the rate of cooling of the article by controlling the wind chill temperature. Consequently, the only control variable in the prior art methods to adjust (lower) the temperature is the introduction of a liquid cryogen into the system. In contrast, utilization of forced gas convection adds a wide range of variable control to adjust the effective temperature, up or down, by controlling the velocity at which the refrigerant is circulated over/around the article to be cooled. Such a forced gas convection system is disclosed by Thomas in U.S. Pat. No. 6,389,828, incorporated herein in its entirety by reference thereto.
The basis of forced gas convection is the principle that increasing velocity of a refrigerant over a heated surface, such as by blowing, greatly enhances the transfer of heat from that surface. In the context of cold temperatures, this principle is probably better known indirectly from the commonly used phrase xe2x80x9cwind chillxe2x80x9d temperature, which is frequently reported on TV or radio by weather announcers. In that context, wind chill temperature is what the temperature outside xe2x80x9cfeelsxe2x80x9d like, taking into account the ambient temperature and the prevailing velocity of the wind. The stronger (higher velocity) the wind, the lower the temperature xe2x80x9cfeels,xe2x80x9d compared to if there were no wind present. Forced gas convection cooling systems, as disclosed herein, take advantage of this xe2x80x9cwind chillxe2x80x9d affect in their ability to remove heat from an object faster with a constant temperature of a gas. In other words, if a 400xc2x0 F. object is placed in a constant 75xc2x0 F. atmosphere without velocity of the surrounding atmosphere, the transfer of energy from the object to the surrounding atmosphere by convection is much slower than if the atmosphere has a velocity over/around the object. An increase in velocity will increase the rate of energy transfer, even though the temperature of the atmosphere is constant. The rate of cooling can be increased or decreased by manipulating the velocity of the cooling medium as the temperature of the medium remains constant. This principle is advantageously utilized to significantly enhance the cooling efficiency of the system by creating, and controlling, xe2x80x9cwind chillxe2x80x9d temperature during the cooling process. As a result, the efficiency of the process is increased while simultaneously reducing the size, which is typically the length, of the cooling system.
However, the previous method disclosed by Thomas utilizes only a measurement of the ambient temperature within the cooling chamber to adjust the velocity and discharge of cryogen. An extrudate leaving a cooling chamber does not necessarily need to be cooled to an even temperature throughout, but may rely on xe2x80x9cequilibrium cooling.xe2x80x9d This principle is advantageously utilized according to the invention to significantly enhance the cooling efficiency of the system by creating and controlling the xe2x80x9cwind chillxe2x80x9d temperature during the cooling process in relation to a measurement of the temperature of the product after leaving the cooling chamber. The basis for xe2x80x9cequilibrium coolingxe2x80x9d is that a mass having two different temperature zones, or a temperature gradient, will exchange energy between the two zones until an xe2x80x9cequilibriumxe2x80x9d temperature is reached. Thus, a manufacturer can reduce cooling time and cooling system length by super-cooling at least 51% of the extrudate mass to form a xe2x80x9cskinxe2x80x9d having sufficient rigidity such that the extrudate may be handled as needed and then allowing the xe2x80x9cequilibrium coolingxe2x80x9d effect to take place after the extrudate has left the cooling system.
Another type of prior art cooling system utilizes a device called a xe2x80x9ccalibrator,xe2x80x9d and typically multiple such calibrators, to cool extrusions. A calibrator is a tool which generally has a central opening through which the extrusion is fed, the central opening having a surface which is generally in contact with the surface of the extrusion as it is fed through. As a result of contact with the surface of the extrusion, the calibrator acts as a heat sink and the heat is conducted to the calibrator and away from the extrusion thus cooling the extrusion. Since cooling of the extrudate tends to make the material contract or change shape, a vacuum generated by external vacuum pumps is generally drawn through grooves in the calibrator inner surface making contact with the extrudate. This vacuum assists in maintaining the shape of the extrudate. To enhance the heat transfer from the extrusion, internal passages or circuits are provided in the calibrator through which a coolant is circulated. Typically, the coolant is water, but liquid nitrogen is also known to have been used to some degree. However, circulating liquid nitrogen through the cooling circuits has met with some difficulties regarding contact of the liquid nitrogen with the calibrators. Additionally, cooling water systems include the inherent problems associated therewith as discussed above. The aforementioned U.S. Pat. No. 6,389,828 to Thomas discloses that it is preferable to first vaporize a liquid cryogen, such as liquid nitrogen, and then to circulate the super-cold vapor/refrigerant through the cooling circuits instead of the liquid cryogen, which thus requires a system for vaporizing the liquid cryogen prior to circulation through the cooling circuits of the calibrator. Although such a method is an improvement over the prior art, the system may still require the use of external vacuum pumps as previously stated. The present invention provides for a calibration tooling chamber utilizing forced-gas convection of a cryogenic refrigerant in combination with a calibrator tooling or sizing template having a plurality of fins in an outer surface thereof to allow the extrudate to be cooled at an effective rate. This eliminates the need for internal passages, and thus the additional manufacturing costs associated with the required set-up/connection/break-down of the equipment between different product runs. Further, the present invention, by use of a forced gas convection cooling chamber, provides a means of generating an internally induced vacuum to assist the extrudate without the requirement of a separate external pump. External vacuum pumps are expensive, require continued maintenance and repair, are noisy and they must be replaced often.
Many extruded articles include at least one hollow, such as pipe, hose, etc., or may contain several hollow portions. Prior art cooling systems provide the manufacturer with only the ability to cool an extrudate from an outer surface thereof by contact with a cooler medium (liquid, gas or solid depending on the system). Depending on the product geometry, however, a significant amount of an extrudate""s mass may be positioned inward of the outer surface and between several hollow portions. Thus, it is difficult to quickly and effectively cool such an extrudate quickly because the cooling medium does not make contact with those portions. The present invention provides an apparatus and method for cooling an extrudate having at least one hollow by circulating a vaporized cryogen through the hollow, preferably in combination with exterior cooling techniques as disclosed in U.S. Pat. No. 6,389,828 and taught herein. This provides for increased cooling capacity and control, as well as reduced cooling system length requirements.
Accordingly, there is a need for a method and apparatus for cooling articles which can provide improved efficiency, reduce the size of the cooling system, and a cooling system that does not require external vacuum pumps.
A method and apparatus for cooling articles are provided which can utilize the dispersion of a liquid cryogen into a feed chamber wherein the liquid cryogen is substantially vaporized and then circulated through a cooling chamber containing the article to be cooled. The vaporized cryogen can be further circulated though the cooling chamber at a controllable velocity, over/around the surface of the article to be cooled and/or tooling, in order to regulate the rate of cooling the article by controlling the wind chill temperature, based upon the principles of forced gas convection.
A presently preferred cryogen is liquid nitrogen. The liquid nitrogen can be dispersed into a feed chamber in a controlled manner using a valve, which can be operated by a controller, such as a microprocessor. Since the temperature in the feed chamber is much higher than the boiling point of the liquid nitrogen, a high BTU (British Thermal Unit) and expansion rate is captured thereby producing an extremely effective refrigerant. The feed chamber can be communicated with a cooling chamber into which the vaporized cryogen can be circulated by a fan, or other device for circulating a gas and/or vaporized cryogen. Either the feed chamber or the cooling chamber can be vented to dissipate pressure generated as the liquid nitrogen rapidly expands to gaseous form. The fan can preferably be a variable speed fan, or other variable speed circulation device, for circulating the vaporized cryogen through the system at a controllable velocity to take advantage of principles of forced gas convection. The fan can be located in the feed chamber to aid in substantially vaporizing the liquid cryogen. However, considering the relatively high temperature utilized in the cooling chamber compared to the boiling point of the cryogen, even without the fan, the liquid cryogen will virtually completely and instantaneously vaporize as it is injected into the feed chamber. The fan can be operated by the controller which can regulate the speed of the fan to provide improved temperature control over the system by controlling the wind chill temperature in the cooling chamber. The system can also include a temperature sensor, connected to the controller, for monitoring the temperature in the cooling chamber, and to calculate the wind chill temperature. An additional external temperature sensor is provided and connected to the controller. The external temperature sensor is adapted to monitor the temperature of an article after the article has exited the cooling chamber and relays the output signal to the controller, which can operate the fan and valve to provide improved temperature control over the system by controlling the wind chill temperature in the cooling chamber in relation to the article""s exit temperature. A heating device can be provided to increase the temperature in the cooling chamber, if needed. The speed of the fan can be controlled by the microprocessor to circulate the refrigerant at a high volume (CFM) to maximize the cooling efficiency, thereby minimizing cryogen consumption. Essentially, the rate of cooling of the article can be increased for a given amount of cryogen dispersed into the feed chamber by increasing the speed of the fan. Another way to express this concept is to say that the xe2x80x9ceffective temperaturexe2x80x9d in the chamber can be reduced by increasing the speed of the fan. The articles to be cooled can be delivered into the cooling chamber by means of a conveyor belt, or various other ways of feeding articles, for example pulling extrusions, through the cooling chambers.
The cooling system can also employ a plurality of cooling chambers, preferably adjacent, each of which can be individually controlled by one or more controllers. The controllers can manage the speed of the fan and the nitrogen injection for each individual cooling chamber, thereby providing for maximum heat exchange rates for efficiency and effectiveness. Each cooling chamber can be equipped with its own temperature sensor, nitrogen injection valve to control the introduction of nitrogen into the cooling chamber, and variable speed fan for circulating refrigerant through the cooling chamber.
In general operation, the temperature sensor detects the temperature in the cooling chamber, or of the circulated refrigerant, and the external temperature sensor detects the temperature of an article that has exited the cooling chamber and each feed the respective information to the controller. The controller can be programmed with a desired temperature to which the temperature inside the cooling chamber is to be regulated or to the desired temperature of the article as it exits the cooling chamber. The controller can also control the nitrogen injection valve and the speed of the fan to cause the temperature in the cooling chamber to correspond to the desired temperature or temperature calculated to cool the article to the desired article temperature. An equation for calculating the xe2x80x9ceffective temperature,xe2x80x9d i.e. wind chill temperature, from the speed of the fan and the ambient temperature in the cooling chamber can be programmed into the microprocessor. The speed of the fan can thus be regulated to increase or decrease the rate of cooling of the article, by adjusting the effective temperature in the cooling chamber, in order to maximize the efficiency of the cooling system. Principles of forced air convection can thus be utilized to increase cooling efficiency while minimizing the consumption of nitrogen. Likewise, principles of forced gas convection can be utilized in combination with principles of xe2x80x9cequilibriumxe2x80x9d cooling to quickly cool surfaces of an article to produce a xe2x80x9cskinxe2x80x9d of sufficient rigidity for further handling. A xe2x80x9cskinxe2x80x9d may be super-cooled (cooled to a temperature below the desired article temperature), but the core remaining at a temperature higher than the desired article temperature. The warmer core regions continue to transfer energy to the cooler xe2x80x9cskinxe2x80x9d regions after exiting the cooling chamber until the two regions reach an xe2x80x9cequilibriumxe2x80x9d temperature. Thus, the cooling systems of the present invention can produce the required cooling with less line space. The fan additionally permits improved system control over the effective temperature in the cooling chamber. A method of cooling an article using xe2x80x9cequilibriumxe2x80x9d cooling according to the invention comprises the following steps: a) introducing liquid cryogen into a feed chamber wherein said liquid cryogen is substantially vaporized; b) circulating said vaporized cryogen from said feed chamber into a separate cooling chamber containing said article to be cooled; c) circulating said vaporized cryogen at a controllable velocity from said feed chamber into said cooling chamber and around said article to create a wind chill temperature in said cooling chamber to increase a rate of cooling of said article; d) sensing the temperature in at least one of said feed chamber and said cooling chamber; e) calculating said wind chill temperature in said cooling chamber, said wind chill temperature being a function of the temperature in said cooling chamber and the velocity at which said vaporized cryogen is circulated through said cooling chamber over said article; f) selecting a desired product temperature; g) sensing the temperature of the article prior to entering said cooling chamber and calculating a difference between said desired product temperature and said temperature of the article prior to entering said cooling chamber; h) calculating an amount of energy that must be removed from said article during the resonance time said article is in said cooling chamber necessary to cool greater than 50% of the mass of said article to a super-cool temperature below the desired product temperature, such that the difference between said super-cool temperature and said desired product temperature is greater than or equal to said difference between the sensed temperature of the article prior to entering the cooling chamber and the desired product temperature, said amount of energy being a function of the heat capacity, thermal conductivity, and resonance time of said article in said cooling chamber; i) calculating a wind chill temperature necessary to remove said amount of energy; and i) controlling said velocity to cause said wind chill temperature to correspond to said wind chill temperature necessary to remove said amount of energy.
Another embodiment of the invention is a cooling system which, utilizing wind chill temperatures, is particularly adapted to vaporize a liquid cryogen and circulate the refrigerant over/pass metal tools for an article within the tool. Specific examples of such tools are a calibrator and a sizing template, which are commonly used to cool extruded articles. The metal tools are provided with a plurality of fins extending from an outer surface thereof that provide for increased external surface area. The metal tools are enclosed within a cooling chamber, or chambers and the metal tools, such as calibrators, through which an extrusion is passed to be cooled, is itself, along with the extrusion, cooled within a cooling chamber. Advantageously, such a system can be vacuum assisted without the need for costly external vacuum pumps. The cooling chamber includes an outlet throat through which refrigerant enters the cooling chamber and an inlet throat through which the refrigerant exits the cooling chamber and is recirculated by a fan. By providing the outlet throat with a cross-sectional area less than the cross-sectional area of the inlet throat, the fan is thus xe2x80x9cstarvedxe2x80x9d and a vacuum is induced within the cooling chamber. Preferably, a restrictor plate or other suitable mechanism is provided that can be operated to vary the cross-sectional area of the outlet throat, inlet throat, or both.
Another embodiment of the invention is a cooling system which, utilizing principles of forced gas convection, is particularly adapted to vaporize a liquid cryogen and circulate the vaporized through a hollow within an extrudate. The cooling system includes similar components as previously discussed, except the vaporized cryogen is communicated to the hollow through an inlet bore provided in an extruder die and mandrel. Preferably, the cooling system is xe2x80x9ccaptivexe2x80x9d and the vaporized cryogen is recirculated. For example, the vaporized cryogen can exit the hollow within a closed cutting chamber. The cutting chamber communicates with a fan via a return conduit. Operation of the system is the same as previously described. Optionally, the cooling system is used in combination with a cooling system to simultaneously cool the outer surface of the extrudate, such as a metal tool cooling system according to the invention.
Other details, objects, and advantages of the invention will become apparent from the following detailed description and the accompanying drawing figures of certain embodiments thereof.